GARN - vacuum relief duct

[Ecky]

In the installation manual (last paragraph pg 8), they talk about a vacuum relief duct. The way I understand it is that the intake (outdoor portion) is to be 3' above ground then rise within the insulation 4' and then terminate (with or without a screen). I am at a loss as to what this thing does? Since the blower has it's own intake, what is the purpose of this ductwork? In diagram A.1, the unit is located on the north wall beside the GARN. Does it have to be? Can it be located on any wall (I assume it can)? Can it be located in front of the GARN in the loading area? (This would not be within the insulated area but would be a lot easier in my case. This goes back to understanding why the thing is necessary in the first place).

I was going to have access to the top through one of the gable ends. This enables me to close and easily insulate above the work area with no access hatch necessary through the insulation. I guess I am not understanding the real purpose of this vent. Why would it show being above the unit if the air is going to be drawn from the work area when the door is opened? Why not just have a vent going through the wall in the work area? Why does it open 4' above the entry point? Lots of questions.

 

[Como]

I just tried to post on the Garn site, but it is having problems.

Also wondering...

 

[varadhammo]

I would like to bump this thread, as I am also unsure of why this vacuum relief duct is necessary. Anybody have an answer?

 

[mike van]

The OP is from 2010, my book from last year has nothing about a vacuum relief duct. No idea what it was or did.

 

[heaterman]

I've never heard of such a thing as described above and have never installed anything like it.

The only thing I can think of is it may be something recommended/used with the optional intake damper.
I should also add that I can't recall ever hearing Martin or any of the guys at Garn mention it either.

 

[Holley]

The OP was referring to a piece of 4" x 14" duct that was shown in some of the "GARN Barn" ("Energy Center") drawings in the 2008 and earlier manuals. The duct was to to be run from a screened intake hood three feet above grade on the outside of he building to a point just below the ceiling inside the GARN room. It's purpose was to supply make up air to the room to ensure that no smoke would enter the room when you opened the GARN loading door during a firing. Leaving the outside door to the GARN room cracked open an inch when you were reloading achieved the same thing.

And that of course is one of the nice features of operating a GARN. Because a draft induction blower pulls air through the combustion chamber , little to no smoke enters the room when you open the door during a firing.

 

[heaterman]

Aha! I was punishing my limited brain cells trying to think of something to do with combustion air for the Garn. Obviously not the case.
Thanks for the clarification Chris.